The GOP needs to win a net of either four seats for an outright majority or three seats and the presidency, in which case a Vice President Paul Ryan would be the tie-breaker in a 50-50 Senate.
So, assuming they lose Maine and win Nebraska and North Dakota, they would need to win two or three of those other toss-up states, depending on the result of the presidential campaign.
Of course, this calculus will change, and several other seats could work their way into the equation.
While Republicans have gotten some good news in recent weeks for two of their most vulnerable incumbents -- Sens. Scott Brown (R-Mass.) and Dean Heller (R-Nev.) -- those seats are hardly off the table. And if Democrats win either, it will be very tough for Republicans to win a majority.
In addition, we're adding two new states this month to our Line of the 10 states most likely to flip control: Connecticut and Indiana. And if they continue to poll like toss-ups, that will throw a wrench in things.
(Those two states were added because we're taking off Missouri and New Mexico. It's become pretty clear that GOP Rep. Todd Akin is staying put in Missouri, which is great news for Democrats; and in New Mexico, Republicans have reportedly canceled their ad buy in a blue-leaning state they had hoped to make competitive.)
To the Line! (As always, these races are rated from most likely to flip -- No. 1 -- to least likely -- No. 10.)
10. Connecticut (Democratic-controlled): This one sticks out on this list like a sore thumb. Do Republicans really have a chance in the strongly blue Nutmeg State? At this point, the answer to that question has to be "yes". As we wrote this week in moving this race from “solid Democratic” to “lean Democratic,” Republicans have renewed hope just two years after their nominee, self-funder Linda McMahon, lost by double digits in another open seat race. McMahon is back, and Republicans think she’s better this time. She’ll need to be. The attacks are coming, and the Democratic Senatorial Campaign Committee just spent its first money here – a sign of the fact that this race is worth keeping an eye on. (Previous ranking: N/A)
9. Indiana (Republican-controlled): Indiana, like Connecticut, is expected to be non-competitive at the presidential level – and also at the gubernatorial level. But every poll we’ve seen has shown the Senate race neck-and-neck. As with Republicans in Connecticut, Democrats see this race as a potential spoiler just two years after their hopes were dashed in another open seat race they talked a big game about. And as with McMahon, there’s a premium on what kind of campaign Rep. Joe Donnelly (D-Ind.) can run. (Previous ranking: N/A)
8. Nevada (R): The Democratic Senatorial Campaign Committee hasn't lost faith in Rep. Shelley Berkley (D) amid her ethics woes. The committee is pouring another $750,000 into advertising, on top of a previously placed $2.3 million reservation. Republicans believe that in a close race, the ethics cloud hanging over Berkley's head could make the difference for Heller. The presidential race also looks very close in this state, so each side's turnout operation will be crucial for both elections. (Previous ranking: 9)
7. Massachusetts (R): Elizabeth Warren's rebooted ad strategy could be just what she needs to turn the race in her favor, but it's also a sign that what she has been doing so far hasn't worked as well as she'd like. Warren's latest ad is a boxing-themed spot that takes aim at Brown's record -- a departure from previous spots which were positive and mainly featured Warren addressing the camera. Brown's own ads, which feature his signature pickup truck, appear to be helping him. Personality could go a long way with the independent voters both candidates need to win, and each side is trying to capitalize. (Previous ranking: 7)
6. Virginia (D): Just about every poll released on this race has shown a neck-and-neck contest, and the latest NBC News/Wall Street Journal/Marist survey is no exception. That poll showed former Democratic National Committee chairman Tim Kaine and former senator George Allen (R) deadlocked at 46 percent. But the same poll showed Obama with a slight lead over Romney. Kaine lagged behind Obama in the poll among non-white voters by 10 points. It's ground he will need to make up in November to put him over the top against Allen. (Previous ranking: 8)
5. Wisconsin (D): Thompson continues to look good in a state that is increasingly looking like a toss-up state at the presidential level. Four polls in August showed him leading Rep. Tammy Baldwin (D) by between five and 11 points. The question is whether that’s simply because he’s a well-known former four-term governor – a resume unmatched by any other Senate candidate. If Romney can come close to winning or carrying the state in November, it will be very hard for Baldwin to beat Thompson. (Previous ranking: 6)
4. Montana (D): Check out the Real Clear Politics average of polling in this race and you'll see why it's so difficult to handicap. Rep. Denny Rehberg takes 49 percent while Sen. Jon Tester (D) takes 48 percent -- a statistical dead heat that's been in place since Rehberg announced he was running. The race tilts slightly to Rehberg -- wethinks -- due solely to the fact that Mitt Romney is likely to carry the Last Best Place convincingly. (Previous ranking: 4)
3. North Dakota (D): Both sides now acknowledge that the open seat race to replace retiring Sen. Kent Conrad (D) is closer than expected. But you've got to remember that this is North Dakota in a presidential year, meaning that former state attorney general Heidi Heitkamp (D) is going to have to run ahead of her national ticket by (at least) 6 to 8 percentage points to win. Rep. Rick Berg (R) is not nearly the quality candidate that then-Gov. John Hoeven (R) was when he cruised to a Senate victory in 2010, but the congressman likely doesn't have to be. (Previous ranking: 3)
2. Nebraska (D): At this point, the only drama left in the race is centered around how much former Sen. Bob Kerrey (D) will lose by. Deb Fischer can start measuring the drapes barring some sort of catastrophic mistake. (Previous ranking: 2)
1. Maine (R): Republicans insist this is not a lost cause and that Secretary of State Charlie Summers could shoot the gap if King -- the heavy favorite -- and Democratic nominee Cynthia Dill split the left-leaning vote enough. But Dill needs to take more than the single digits she’s managing in early polls of the general election. Conservative groups have started spending some money hoping to move the numbers. If they can’t do it in the coming days and weeks, though, expect them to pull out, with King being on pace for a coronation. (Previous ranking: 1)
Sean Sullivan and Chris Cillizza contributed to this report.